The present invention relates generally to a twist drill and more particularly to a multi-surface drill point geometry.
A drill is a rotating-end cutting tool for creating or enlarging holes in a workpiece of solid material. A drill is also known as a drill bit. A twist drill is a drill having one or more spiral or helical grooves or flutes, extending from the point to the smooth part of the shank, for rejecting cuttings. The portion of the twist drill along which the flutes spiral is called the barrel. Between the flutes are wing shaped blades connected along the axis of rotation of the twist drill by the web portion of the barrel. The flutes spiral in a direction such that the material cut by the twist drill is carried away from the point through the flutes along the barrel to be expelled upon reaching the surface of the workpiece.
Most of the cutting work of a twist drill is accomplished at the end of the barrel referred to as the point of the twist drill. The spiraling leading edges of the wing shaped blades extending along the barrel form the margins that provide side support and centering of the drill in the hole being cut. The drill point experiences severe stress and heat conditions in use. Optimally, the twist drill should perform so as to drill a hole straight into the workpiece and not wander and change direction, to have a reasonable lifetime and to not tend to fail due to fracturing or to exhibit excessive wear of the cutting edges.
The leading edge of the wing shaped blade at the point of the drill does most of the cutting work of the drill and acts to slice through the bottom of the hole in the workpiece shaving off a sliver of material of the workpiece. This sliver is formed in a spiral and tends to come off in a curl shape, due in part, to the differences in its length from its inside edge which is being cut from the center of the hole to its outside edge which is being cut from the cutside of the hole. Because of the forces on this sliver it tends to break after it gets to a certain length, forming a chip. The formation of these chips causes wear in the cutting edges of the twist drill. It is known to form the point surfaces of twist drills such that the point cutting edges are varied in depth in a non-linear fashion to an extent so as to break up the chip or to create narrow chips. This kind of construction may, if properly designed, decrease wear on the point cutting edges and may also enable cutting with less torque being applied to the twist drill.
Another problem encountered by twist drills, as mentioned above, is the ability of the drill to maintain its direction and drill a straight hole and not wander off to the side. In typical standard twist drills, at the point of the drill where the faces of the cutting blades come together, a ridge is formed that is referred to as the chisel. The greater the chisel width, the greater the tendency of the twist drill to wander and not drill straight through the material. Furthermore, as the chisel width increases, there is an increased area of the center of the hole being drilled that is not being cut by a cutting surface but, rather, is being extruded and forced away by the chisel surface. Greater force is required to remove material by this chisel action than is required to remove it by slicing it with a cutting edge. Therefore, as the chisel width increases, both the axial force and the time required to drill a hole tend to increase. In a number of known configurations to minimize the chisel width, a portion of the point surface at the trailing and non-cutting edge of the cutting blade is ground away from near the center of the twist drill, at its axis of rotation, out to the periphery of the drill, thus eliminating most of the chisel width. This type of twist drill is commonly referred to as a split point twist drill. See: Cutting Tool Engineering, May 1968, page 16; American Machinist, January 1982, page 55; U.S. Pat. Nos. 2,936,658; 1,467,491; 4,222,690; 4,381,161; 4,456,411; 4,605,347. The split point twist drill cuts easily through material, having eliminated a substantial portion of the chisel, however known designs have been prone to be short-lived, suffering fracture failure.
Many attempts have been made to design a multi-surfaced drill. In Clement, U.S. Pat. No. 4,456,411, a groove is cut in the cutting face of a standard drill. Also, many attempts have been made by the Chinese over the past thirty years to develop a multi-surfaced, sometimes referred to as multi-faceted, twist drill that exhibits all the desirable characteristics discussed above. Some multi-surfaced point designs have been found to exhibit superior qualities. Unfortunately these designs because of their complexity, have not been economically manufacturable. Indeed, the superior Chinese design multi-surfaced drills are made by hand.
Attempts at improving the drill point face geometry have extended over many decades and numerous variations of face geometries have been tried. Yet a superior multi-surfaced drill design has not emerged which can be economically mass-produced.